In the field of space frames, such as may be used to support roofs over a large span, of vital importance is the strength of elements used to connect between a number of structural elements at a point in space and the way in which the connection is made. The most common geometrical configuration of space frames is the double layer, square module frame, utilizing a single type of connectors. Such a typical connector is designed to connect eight frame elements, the first four of which are substantially coplanar, and the second four are inclined relatively to and on the same side of the plane defined by the first four. The first four elements may be referred to as `the primary frame members`, since they usually experience the more severe tensile or compressive loads. As will be appreciated by persons skilled in the art, it would be desirable for the elements to meet at a single point defined by the axes along which they lie.
In practice, however, the way in which a connector is designed to receive each of the elements may allow eccentricity in the connection, giving rise to local moments within the space frame. Where the connector is configured to define an almost-entirely non-eccentric connection, the connector is usually very expensive to manufacture, primarily due to high strength requirements: in all concentric connectors, each of the connecting frame elements conveys its full load to the connector base, therefore it must be sufficiently strong to withstand a load which equals at least to that of the most severely stressed frame element.
Made by MERO Tube Construction & Accessories of Wurzburg 4, P.O.B. 462, West Germany is a ball connector having up to nine pairs of holes aligned along up to nine different axes. Each hole is provided with a screw thread and is made so as to receive a structural element, the end of which has a screw configured to be screwably retained by the hole.
A disadvantage of this connector is that two members, for example, lying along a common axis, are not self-locked by means of the connector. Therefore, as movement occurs through the frame, such as may be generated by wind or other conditions of dynamic loading, there is a danger that at least one of the two members will slowly start to unscrew from the connector and that, over a period of time, the member will become completely detached and thereby contribute to failure of the entire structure of which it forms a part. To prevent such danger, relatively high level of periodic maintenance is required. Furthermore, due to the precision machining manufacturing process of the connector, it is generally relatively expensive.
Made by BEN-EZRA KARAGOLA METALWORKS LTD., P.O.B. 805, Industrial Area, Kfar Saba, Israel, is a model B.E.K. connector for space frames. It comprises eight members for receiving tubular structural elements, a first four of which are arranged along two perpendicular axes in a common plane and a second four of which are arranged for partial rotation within two mutually perpendicular planes, each of the second four being arranged so as to receive a diagonal member lying along an axis intersecting with the common plane.
Among disadvantages of the connector to Ben-Ezra is that each of the members is associated with a base member by means of a single bolt shear connection. The holes through which the bolt passes are provided with a relatively high tolerance, for example, a hole of 14 mm diameter is conventionally provided for a 12.7 mm shank diameter bolt so as to ease construction.
As all of the members, including the first four, which would normally experience the more severe loading conditions, are connected by bolts, as outlined above, a large span space frame using the Ben-Ezra connector would, therefore, be liable to sag due to the relative freedom of movement of the bolts within their holes. Such a frame would also be susceptible to high amplitude movement under dynamic loading conditions, such as wind, therefore making the space frame liable to early failure.
Marketed by U. SHLEISSNER LTD., Shiffer St., Petach Tikva, Israel, is a STRUCTIC connector for space frames. The connector defines a support configured to be mounted onto a first primary frame member, made of continuous tubing, at a junction of the member with a similar second primary member and with up to four diagonal members. The support defines a first generally semicircular cutout for mounting onto the first member and a second semicircular cutout for receiving the second member. The first and second tubular primary members are connected to the support by means of field welding during assembly of the structure. The support also includes up to four tongues, generally bisecting the right angles between the primary members and inclined relatively to the planes defined thereby, so as to receive and weldably engage the diagonal members.
Among disadvantages of the Structic connector is that a relatively large eccentricity, approximately equivalent to the diameter of one of these tubular frame members, exists between the first and second primary frame members. This eccentricity leads to the undesirable build-up of relatively large moments within a space frame in which the Structic connector is used.
Furthermore, as the support is attached to the first and second members by on-site welding, thereby preventing application of high quality protective coatings to the welds, they are prone to corrosion. This connector also requires, therefore, a relatively high level of maintenance.
Disclosed in U.S. Pat. No. 3,507,526 to Packman et al is a joint between tubular members lying substantially in a common plane in a tubular steel space structure. The joint is made by means of a pair of clamping elements which are held together by a bolt or bolts and between them define sockets formed with alternating transverse recesses and projections for the reception of the complimentarily shaped ends of the tubular members.
Disclosed in U.S. Pat. No. 4,355,918 to Van Vliet is a connector for elongated frame elements of a structural space frame. The connector consists of three elements which are stacked one on the other along a common axis with the face of each element which abuts that of another including recessed formations which between the opposite element faces define sockets for trapping the headed ends of space frame members. The axes of the sockets between a first pair of element faces lie in a common plane and those between the second pair of faces are inclined relatively to and on the same side of the plane containing the axes of the sockets between the first pair of element faces and apparatus for clamping the elements in the direction of their common axis.
Both the joint to Packman et al and the connector to Van Vliet are characterized by their complex structures and the resulting complicated assembly, resulting in relatively high costs of manufacture when compared with less complex connectors.
The joint to Packman et al is also characterized, in some cases, by welding of members after their insertion between the clamping elements. This welding may cause the connection to be susceptible to corrosion.
The connector to Van Vliet calls for the structural elements being connected to be formed with narrow neck portions at the connection end. The manufacturing process by which the narrow neck portions are formed is, as known in the art, a relatively expensive process.
Further connectors and/or structures employing connectors are disclosed in the U.S. Pat. Nos. 1,771,362, 2,149,844, 4,236,642 and 4,637,748; and in the Israel Patents Nos. 19,782, 29,037 and 69,867.